1. Field of the Invention
The present invention relates to a semiconductor device using a heterojunction bipolar transistor (hereinafter, described as HBT) and a method of fabricating the semiconductor, particularly to a semiconductor device for a power amplifier for a mobile communicating machine and a method of fabricating the semiconductor device.
Further, the present invention relates to a power amplifier reducing temperature dependency of power gain and enabling high power conversion efficiency.
2. Related Art
In recent years, with rapid growth of demand of a mobile communicating machine, research and development on a power amplifier used for the communicating machine has intensively been carried out. Examples of semiconductor transistors used for a power amplifier for a mobile communicating machine include GaAsHBT, GaAs field effect transistor (hereinafter, described as FET), and SiMOS (Metal-Oxide-Semiconductor) FET. Among them, GaAsHBT is centrally used as a transistor for a power amplifier for a mobile communicating machine since the transistor is provided with characteristics of being excellent in linearity of an input/output characteristic, operated only by a positive power source, dispensed with circuit or part required for generating a negative power source, having a high output power density and having a small chip area to thereby save space and reduce cost.
It is indispensable to reduce base/collector capacitance per unit emitter area to promote performance of a power amplifier using GaAsHBT, particularly to promote power added efficiency, power gain and the like. For that purpose, it is necessary to reduce a ratio r of a base/collector junction area per an emitter/base junction area. A collector top HBT structure is generally known as a method for reducing r and the structure is disclosed in IEEE Transaction on Electron Devices Vo. 42 No. 11 (1995) pp.1897-1902).
FIG. 3 shows a vertical sectional structure of a collector top HBT fabricated by the prior art noted above. As is apparent from the figure, r=1 can be realized, however, in comparison with an emitter top HBT structure, there are increased steps of forming a parasitic emitter/base high resistance region 35, specifically, steps of ion-implanting boron or the like and annealing, the fabricating method becomes complicated and therefore, there poses a problem which amounts to an increase in cost of a semiconductor device.
Therefore, in order to promote the performance of a semiconductor device while avoiding an increase in cost, it is pertinent to achieve a reduction in r by changing layout of the emitter top HBT. In the case of HBT of the prior art using a rectangular emitter shape, it has been difficult to realize r<2.5. This is because a base electrode and through holes connecting the base electrode and wiring restruct a rate of reducing the base/collector junction area. In contrast thereto, r can be reduced by constituting the base electrode only by a base through hole region and changing a planar shape of emitter/base junction from the conventional rectangular shape to a ring-like structure. Such a ring-like emitter structure has been proposed in an Si bipolar transistor avoiding an erroneous operation of software error or the like caused by irradiation of α-rays and disclosed in JP-A-5-3204. Although in the prior art of the Si bipolar transistor, the collector electrode is arranged on an inner side of the ring-like emitter/base junction, as shown by a plan structure view of FIG. 1, r<2.5 can be realized by arranging the base electrode in place of the collector electrode and arranging the collector electrode on an outer side of the ring-like emitter/base junction.
It is a first object of the present invention to provide a semiconductor device using HBT satisfying r<2.5.
It is a second object of the present invention to provide a semiconductor device using HBT satisfying r<2.0.
It is a third object of the invention to provide a method of fabricating HBT satisfying r<1.5.
It is a fourth object of the present invention to provide a semiconductor device having a high power added efficiency and a high power gain and being suitable for a power amplifier.
Next, an explanation will be given of a situation heretofore with regard to a power amplifier. In recent years, with rapid growth of demand of mobile communicating apparatus, research and development on a power amplifier used in a communicating machine has intensively carried out. Examples of semiconductor transistors used in a power amplifier for a mobile communicating machine include a heterojunction bipolar transistor (hereinafter, abbreviated as HBT), a field effect transistor (hereinafter, abbreviated as FET), and SiMOS (Metal-Oxide-Semiconductor) FET. Among them, HBT is provided with characteristic of being excellent in linearity of an input/output characteristic, operated only by a positive power source, dispensed with circuit or part for generating a negative power source, having a high power output density and a small chip area to thereby save space and reduce cost. Therefore, the transistor is centrally used as a transistor for a power amplifier for a mobile communicating machine.
To realize high performance formation of a power amplifier for a mobile communicating machine, high performance formation of HBT constituting a basic device thereof is indispensable. For that purpose, it is necessary to reduce base/collector capacitance. A technology of using HBT having a ring-like emitter shape is known as a means therefor in, for example, JP-A-2001-189319.
The technology disclosed in JP-A-2001-189319 has a difficulty of high temperature dependency of power gain. FIG. 41 shows a result of measuring temperature dependency of power gain of HBT. FIG. 41 shows respective results of HBT having a ring-like emitter (hereinafter, abbreviated as ring-like emitter HBT) and HBT having a rectangular emitter (hereinafter, abbreviated as rectangular emitter HBT). In both of the ring-like emitter HBT and the rectangular emitter HBT, a total emitter area is designed to be equal to about 800 μm2. Further, the ring-like emitter HBT is constituted by connecting basic HBT having an emitter area of 132 μm2 in 6 rows in parallel and the rectangular emitter HBT is constituted by connecting basic HBT having an emitter area of 108 μm2 in 8 rows in parallel. Measuring conditions are such that collector voltage is 3.5 V and frequency is 1.9 GHz. The respective temperature coefficients of power gain of the ring-like emitter HBT and the rectangular emitter HBT are −0.012 dB/° C. and −0.006 dB/° C. Therefore, when a power amplifier having two stages constitution is formed, in the case of the ring-like emitter HBT, the temperature coefficient of power gain is −0.024 dB/° C. and in the case of the rectangular emitter HBT, the temperature coefficient of power gain is −0.012 dB/° C.
A difference of the temperature dependencies of power gain of the ring-like emitter HBT and the rectangular emitter HBT is caused by temperature dependency of base resistance. The base resistance is increased with temperature rise. Meanwhile, the base resistance of the ring-like emitter HBT is larger than that of the rectangular emitter HBT. Therefore, an amount of change in the base resistance with a change in temperature of the ring-like emitter HBT becomes larger than that of the rectangular emitter HBT. As a result, in the case of the ring-like emitter HBT, mismatch with a matching circuit in view of high frequency is increased with temperature rise and the temperature dependency of power gain is also increased.
Based on the background, it is a fifth object of the present invention to provide a high performance power amplifier having low temperature dependency of power gain.
In addition thereto, it is a sixth object of the present invention to provide a method of fabricating a high performance power amplifier having low temperature dependency of power gain.